Improvements to these steel products which increase their strength, toughness, etc. have been attempted, but the improvements are not uniform in the thickness direction of a steel product and are not uniform among the steel products.
The ability of such products to withstand an earthquake is of particular importance. "Tetsu to Hagane (Iron and Steel)," Vol. 74, No.6, 1988, pages 11 to 21, reports that as buildings get taller, they are being designed to prevent collapse during an earthquake by absorbing the vibrational energy. That is, building collapse is prevented by the plastic deformation of the structural materials. For a building to be designed to show this behavior, the designer must understand the yield point ratios of the steel products of the building.
Accordingly, it is very important that the steel products used in the building, such as steel plates, beams, etc., are homogeneous, showing little variation in the strength.
Steel products used for buildings, shipbuilding, etc., are also required to have high tension and high toughness, and thus the steel products of this kind are usually produced by the TMCP (Thermo-Mechanical-Controlled-Rolling-Process) method in which rolling and cooling are controlled.
However, when a thick steel product is made by the TMCP method, the cooling rate may not be constant during cooling treatment following rolling. This may cause the steel product to vary in quality in the thickness direction or may cause differences in the quality among steel products. By way of example, quality varies in the thickness direction of a thick steel product, there may be significant differences between the characteristics of a web and a flange in a H shaped steel.
The following references are examples of attempts to improve the uniformity of the quality of steel products.
JP-A-63-179020 ("JP-A" means an unexamined published Japanese patent application) discloses a method of reducing the hardness difference in the thickness direction of a steel plate by controlling the components of the steel, the rolling reduction, the cooling rate and the cooling-finishing temperature.
However, in the production of thick steel plates, particularly steel plates more than 50 mm thick, cooling rate changes in the thickness direction of the steel plate are inevitable, so that it is difficult to sufficiently control the difference in hardness in the thickness direction of the steel plate by the method described above.
JP-A-61-67717 discloses the use of very low-C steel to attempt to control the difference in strength in the thickness direction of a steel plate, but as shown in FIG. 3 therein the variation of strength accompanying the change of the cooling rate cannot be avoided in very thick steel plates.
JP-A-58-77528 discloses a steel containing Nb and B in which a stable hardness distribution is obtained. The cooling rate must be controlled to the range of from 15 to 40.degree. C./second to make the structure bainite. However, because it is difficult to strictly control the cooling rate in the central portion of the thickness of the steel plate, a uniform structure is not obtained in the thickness direction of the steel plate so that the strength is uneven and island-form martensite forms which degrades ductility and the toughness.
JP-A-54-132421 discloses a technique for improving welding properties in which a high-tension bainite steel is produced by using a very low carbon content and also by rolling the steel at a finishing temperature of 800.degree. C. or lower to obtain a tough product suitable for line pipe. However, rolling is finished at a low-temperature so that productivity is low. Further, when a thick steel plate is to be cut to a definite length, the cutting may cause a strain.
In JP-A-8-144019, the present inventors have proposed steel products having more uniform quality in which a very low carbon content is used. These products also have excellent shock resisting characteristics of a welding heat influencing portion (HAZ) at 0.degree. C. However, even in these steel products the shock resisting characteristics of the welding heat influencing portion (HAZ) are not always good at a temperature of -20.degree. C., and thus further improvements are desired.